After doing some research on this matter i have come up on the following information. The pictures enclosed and the writing was taken from OPEFE, and a different UK Piranha forum.
Please note, The pictures show both adults and juvenile forms of pygocentrus caribe. Showing the lack of humeral spot, and pale stomach regions. As the text states, it depends on water quality and diet, but i would estimate in upwards of 70 to 80mm..
Juveniles and adults collected in Rio Apuré of Venezuela (UMMZ 214742), dorsal and lateral parts of body from black humeral spot to anal-fin origin silvery gray. Abdomen from lateral spot to anal-fin origin red to reddish-orange. Most of head gray, suffused with orange or red, especially posterior and ventrally. Eye silvery, with black pigments above and below iris. Lower jaw dark anterior, posterior the same red or orange as the abdomen, pectoral and pelvic fins, red through orange. Dorsal fin dark grayish-black. Adipose fin black proximally, often some hyaline area distally. Caudal fin dark grayish-black, with a pale subterminal band or posterior border. Anal fin is black, proximally in the area of small scales on the fin and with black pigment scattered along distal margin; otherwise, fin red or red-orange.
Large juveniles to adults: Based on specimens between about 80-120 mm SL (UMMZ 211299 from the lower Orinoco) much of the color pattern remains similar to that of juveniles. The distinctive changes noted is the loss of the caudal spot, a pale belly compared with the dorsum in larger specimens, and the presence of a humeral blotch just posterior to the opercle. The black blotch on the shoulder reaches dorsally nearly to the dorsal border of the opercular opening and ventrally nearly to the pectoral fin in some specimens. The anterior border of the blotch is the cleithrum (Fink 1993). Specimens at around 80 mm are still densely spotted. In larger specimens, body spotting is no longer present. Very large adults are darker, to the point that the shoulder blotch is not visible. Breeding adults give the appearance of a "black" piranha in coloration.
The extent and hue of the belly coloration is somewhat variable individually, geographically, and depending on color of the water in which the fish lives (FINK, 1993). Pygocentrus cariba is widely distributed in the Rio Orinoco basin lowlands, in Columbia and Venezuela. It is listed among record species, with a 1-pound 4-ounce fish being the largest caught (source: Field and Stream 2002).
